Previously, as a DC-DC converter, for example, of an electronic device, a DC-DC converter of a chopper type that can hold output voltage in a constant range if input voltage fluctuates has been used.
As an example of this kind of DC-DC converter, the following has been proposed: A step-down copper transistor and a step-up chopper transistor are connected though a common choke coil. When the input voltage is higher than the output voltage, the step-up chopper transistor is turned off, and on/off control of the step-down chopper transistor is performed for decreasing the input voltage. When the input voltage is lower than the output voltage, the step-up chopper transistor is turned on, and on/off control of the step-down chopper transistor is performed for increasing the input voltage. (See, e.g., Japanese Laid-Open Patent No. 18970/1987.)
In this DC-DC converter, a DC-DC converter of the step-down chopper type and a DC-DC converter of the step-up chopper type are implemented as the same circuit.
In the example described above, the step-down chopper transistor and the step-up chopper transistor are selectively operated in response to the input voltage. However, when the voltage decrease and voltage increase are switched, the operation of the step-down chopper transistor and the operation of the step-up chopper transistor are not switched smoothly and thus it is difficult to hold the output voltage constant.
A delay occurs during switching in the transistors used as a step-down chopper transistor and a step-up chopper transistor. Therefore, when the step-down chopper transistor and the step-up chopper transistor are operated selectively in response to the input voltage at the switching time of the voltage decrease and voltage increase, the step-down chopper transistor may remain on and the step-up chopper transistor may maintain off. Thus, it is difficult to hold constant the output voltage relative to fluctuation of the input voltage during switching of a voltage decrease and voltage increase.